The present application claims priority to Japanese Application No. P11-235760 filed Aug. 23, 1999, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a semiconductor device having a photodetector and an optical pickup system using the semiconductor device.
A photodiode as a photodetector capable of converting a light signal into an electric signal has been extensively used for optical sensors for controlling various kinds of photoelectric converters, for example, an optical sensor for obtaining a recording information signal (hereinafter, referred to as xe2x80x9cRF signalxe2x80x9d), a tracking error signal, a focusing error signal, and the like in a so-called optical pickup system for recording and/or reproducing light signals on and/or from an optical recording medium.
A photodetector is formed, together with various circuit elements such as a bipolar transistor, resistance, and capacitor, on a common semiconductor substrate, to be thus configured as a so-called photo-IC (Integrated Circuit). Such a photo-IC is generally fabricated in accordance with a method of fabricating a bipolar transistor as one of the above-described circuit elements.
As a photo-IC having a fast, high-sensitive photodetector, there has been proposed a photo-IC including a high resistance expitaxial semiconductor layer.
FIG. 5 is a schematic sectional view showing a prior art photo-IC on which a photodiode PD as a photodetector and a bipolar transistor TR are mixedly formed. The photo-IC shown in FIG. 5 has a configuration of a bipolar IC on which an npn type transistor TR and an anode common type photodiode PD are formed on the same semiconductor substrate 1.
The method of fabricating the bipolar IC will be described below. A high impurity concentration p-type buried layer 3 is formed on the entire principal plane of a p-type Si semiconductor base substrate 2, and a low impurity concentration p-type first semiconductor layer 31 for forming an anode region 4 of the photodiode PD is formed on the buried layer 3 by epitaxial growth. A high impurity concentration collector buried region 5 is formed on a transistor TR formation area of the first semiconductor layer 31. High impurity concentration buried isolation regions 6 are selectively formed in order to isolate circuit elements from each other and to divide the photodiode PD into parts (as will be described below). A high impurity concentration p-type buried region 8 is formed, simultaneously with the formation of the buried isolation regions 6, under a contact of an anode electrode 7 of the photodetector PD.
A low impurity concentration n-type second semiconductor layer 32 for forming a cathode region 9 of the photodiode PD and a collector region 10 of the transistor TR is formed on the first semiconductor layer 31 by epitaxial growth.
In this way, the first and second semiconductor layers 31 and 32 are formed on the semiconductor base substrate 2 by epitaxial growth, to form an Si semiconductor substrate 1. Insulating isolation layers 11 made from SiO2 are formed, by a so-called LOCOS (Local Oxidation of Silicon), on the surface of the Si semiconductor substrate 1, that is, on the second semiconductor layer 31 in order to electrically isolate semiconductor circuits elements or regions from each other.
In the second semiconductor layer 32, a high impurity concentration p-type isolation region 12 is formed between the insulating isolation layer 11 and the buried isolation region 6 positioned thereunder at each insulating isolation portion between adjacent circuit elements. A high impurity concentration p-type anode electrode extraction region 13 is formed on the high impurity concentration buried region 8, and a high impurity concentration anode contact region 14 is formed on the anode electrode extraction region 13. A high impurity concentration p-type division region 30 is formed on the buried region 6, which is formed at the division region for dividing the anode region 4 into two parts, in such a manner as to be in contact with the region 6.
A high impurity concentration n-type collector electrode extraction region 15 and a p-type base region 16 are formed in the collector region 10. An n-type emitter region 17 is formed on the base region 16.
A high impurity concentration cathode region 18 is formed on each cathode region 9 of the photodiode PD, and a cathode electrode 19 is in ohmic-contact with the cathode region 18.
An insulating layer 21 made from SiO2 is deposited on the surface of the semiconductor substrate 1, and electrode contact windows are formed in the insulating layer 21. An emitter electrode 20E, a base electrode 20B, and a collector electrode 20C of the transistor TR are brought into contact with the regions 15, 16 and 17 through the electrode contact windows, and an interlayer insulating layer 22 made from SiO2 is formed thereon. A light shading layer 23 made from Al, which has a light receiving window, is formed on the interlayer insulating layer 22, and a protective layer 24 is formed thereon.
The photodiode PD is irradiated with a light ray to be detected through the light receiving window of the light shading layer 23. In this case, the insulating layers 21 and 22 act as a reflection preventive film.
The photodiode PD configured as the bipolar IC thus fabricated is used as a sensor for obtaining an RF signal, a tracking error signal, and a focus error signal in an optical pickup system for recording and/or reproducing light signals on and/or from an optical recording medium.
FIG. 6A shows a plane pattern of a photodiode PD used as a sensor for obtaining an RF signal, a tracking error signal, and a focus error signal in an optical pickup system. In this example, the photodiode PD includes a central photodiode PD0 divided into four parts A, B, C and D in a cruciform and side photodiodes PDS1 and PDS2 disposed on both the sides of the central photodiode PD0. Such a photodiode PD is irradiated with light from an optical recording medium, typical, an optical disk in such a manner that a central light spot SP0 is formed on the central photodiode PD0, and side spots SPS1 and SPS2 are formed on the side photodiodes PDS1 and PDS2, respectively. In this case, assuming that the outputs obtained by photoelectric conversion at the four divided parts A, B, C and D of the central photodiode PD0 are taken as outputs A, B, C and D, the focus error signal is obtained by calculating an equation of (A+C)xe2x88x92(B+D), and assuming that the outputs from the side photodiodes PDS1 and PDS2 are taken as outputs E and F, the tracking error signal is obtained by calculating an equation of (Exe2x88x92F), and the signal readout signal, that is, RF signal is obtained by calculating an equation (A+B+C+D).
FIG. 6B shows another example of a photodiode PD applied to an optical pickup system. In this example, the photodiode PD includes a photodiode PD1 divided in parallel into four parts A, B, C and D in which the center side divided parts B and C are each formed into an extremely thin stripe pattern with a pitch of 14 xcexcm, and a photodiode PD2 divided in parallel into four parts Axe2x80x2, Bxe2x80x2, Cxe2x80x2 and Dxe2x80x2 in which the center side divided parts Bxe2x80x2 and Cxe2x80x2 are each formed into an extremely thin stripe pattern with a pitch of 14 xcexcm. Such a photodiode PD is irradiated with light in such a manner that a light spot SP1 is formed on the photodiode PD1 and a light spot SP2 is formed on the photodiode PD2. In this case, assuming that the outputs from the divided parts A, B, C and D of the photodiode PD1 are taken as outputs A, B, C, and D and the outputs from the divided parts Axe2x80x2, Bxe2x80x2, Cxe2x80x2 and Dxe2x80x2 of the photodiode PD2 are taken as outputs Axe2x80x2, Bxe2x80x2, Cxe2x80x2 and Dxe2x80x2, the focus error signal is obtained by calculating an equation [(B+C)xe2x88x92(A+D)]xe2x88x92[(Bxe2x80x2+Cxe2x80x2)xe2x88x92(Axe2x80x2+Dxe2x80x2)]; the tracking error signal is obtained by calculating an equation of (A+B+Cxe2x80x2+Dxe2x80x2)xe2x88x92(C+D+Axe2x80x2+Bxe2x80x2); and the RF signal is obtained by calculating an equation of (A+B+C+D)+(Axe2x80x2+Bxe2x80x2+Cxe2x80x2+Dxe2x80x2).
A semiconductor device having a photodiode divided into a plurality of parts, for example, the above-described photodiode PD divided into four parts has a configuration shown in FIG. 7. FIG. 7 is a sectional view of an essential portion of the photodiode. As shown in this figure, a cathode region 9 is divided into two parts over the entire thickness by a division region 30 and a buried isolation region 9 formed under the division region 30.
According to the above-described prior art configuration, in a non-operated state in which no reverse bias voltage is applied to the photodiode PD, the cathode region 9 is perfectly divided into two parts by the division region 30 and the buried isolation region 6 formed under the division region 30. That is to say, the p-n junction J between the anode region 4 and the cathode region 9 is divided into a plurality of junctions Jn by a plurality of the division regions 30 and the buried isolation regions 6. When a reverse bias voltage is applied to the photodiode PD for operating the photodiode PD, depletion layers are extended from each of the divided p-n junctions Jn and from each of the p-n junctions j between the division layers 30 and cathode region 9. In FIG. 7, chain lines xe2x80x9caxe2x80x9d and xe2x80x9caxe2x80x2xe2x80x9d each designate the extension of the depletion layer. It should be noted that in FIG. 7, parts corresponding to those shown in FIG. 5 are designated by the same characters and the overlapped explanation thereof is omitted.
As shown in FIG. 7, the extension of the depletion layer from each of the divided p-n junctions Jn to the anode region 4 side is shallower than the buried isolation region 6, and therefore, the depletion layer is divided into two parts by the buried region 6.
In the case of the semiconductor device including a photodiode having a configuration shown in FIGS. 6A or 6B, that is, so-called photo-IC, in which light spots are formed on the divided parts A, B, C and D or Axe2x80x2, Bxe2x80x2, Cxe2x80x2 and Dxe2x80x2, that is, on each division region 30 and the buried isolation region 6 formed thereunder, the frequency characteristic of the photodiode is degraded for the following reason:
The frequency characteristic of the photodiode is mainly determined by a CR time constant depending on a parasitic capacitance (C) and a parasitic resistance (R), a time required for carriers to migrate in a depletion layer of the photodiode, and a time required for carriers to diffuse in a non-depleted semiconductor layer.
Accordingly, in the above-described photodiode PD divided into four parts, the frequency characteristic differs between a portion near the division region 30 and the buried isolation region 6, and a position sufficiently apart therefrom.
This will be more fully described with reference to FIG. 7. Minority carriers, that is, electrons xe2x80x9cexe2x80x9d generated in the buried isolation region 6 and a portion, near the region 6, of the anode region 4 by light irradiation receive, from the region 6, forces in the direction in which the electrons xe2x80x9cexe2x80x9d are separated from the region 6 as shown by an arrow xe2x80x9cbxe2x80x9d because the potential of the region 6 acts as a barrier against the electrons xe2x80x9cexe2x80x9d, minority carriers. As a result, the electrons xe2x80x9cexe2x80x9d generated in the buried isolation region 6 and a portion, near the region 6, of the anode region 4 migrate toward the depletion layer not along a straight line but along a curved line. On the contrary, electrons xe2x80x9cexe2x80x9d generated in a portion sufficiently apart from the buried isolation region 6 are not affected or little affected by the potential of the region 6, and therefore, they migrate to the depletion layer along a straight line as shown an arrow xe2x80x9ccxe2x80x9d. That is to say, electrons generated in the buried isolation region 6 and its neighborhood are longer than electrons generated at a portion sufficiently apart from the region 6 in terms of migration distance to the depletion layer, that is, diffusion time of carriers. As a result, the frequency characteristic of the buried isolation region 6 and its neighborhood is degraded.
Accordingly, in the case of the above-described photodiode divided into four parts, when a light spot is formed on an area including the isolation regions between the four divided parts, that is, the buried isolation regions 6, the ratio of the area of the isolation regions to the light irradiation area becomes large, thereby causing a problem associated with the frequency characteristic. In particular, since the RF signal is obtained by adding signals outputted from the divided parts of the photodiode including the regions 6, it is affected by the degradation of the frequency characteristic at the regions 6. The RF signal most required to ensure a high speed performance is seriously affected by the degradation of the frequency characteristic at the regions 6.
The light receiving sensitivity of a photodiode is determined by a ratio of those, reaching a depletion layer, without generation of recombination, of carriers (electron-positive hole pairs) generated by photoelectric conversion.
The light receiving sensitivity at the isolation portion of a photodiode will be examined. Referring to FIG. 7, incident light at the isolation portion of the photodiode enters the division region 30, buried isolation region 6, and anode regions 4, 3 and 2. In the division region 30 and the buried isolation region 6, which are positioned near the surface of the semiconductor substrate and thereby have a large light absorption, since the impurity concentration is high, the diffusion lengths of carriers generated in the vicinity of the regions 30 and 6 are short, so that there is a large possibility that the carriers could be lost by recombination before reaching the depletion layer.
The carriers generated in the anode regions 4, 3 and 2 are longer in migration distance to the depletion layer as described above, there is a large possibility that the carriers might be lost by recombination during migration.
As a result, the light receiving sensitivity is degraded at the isolation portion of the photodiode.
Accordingly, in the case of the above-described photodiode divided into four parts, when a light spot to be detected is formed on an area including the isolation regions between the four divided parts, that is, the division region 30 and buried isolation regions 6, since the ratio of the vertical sectional area of the division and isolation regions to the light irradiation area becomes large and further the width of each isolation region may be sometimes wider on the basis of the design of an optical pickup, the degradation of the light receiving sensitivity in the division or isolation region causes a large problem.
An object of the present invention is to provide a photodetector, typically, a photodiode capable of improving the frequency characteristic even if the photodetector is configured such that a division portion and its neighborhood is irradiated with light.
Another object of the present invention is to provide a photodetector, typically, a photodiode capable of improving the light receiving sensitivity even if the photodetector is configured such that a division portion and its neighborhood is irradiated with light.
To achieve the above objects, according to a first aspect of the present invention, there is provided a semiconductor device including a photodetector having a junction at which a first conductive type first semiconductor portion and a second conductive type second semiconductor portion are joined to each other, the photodetector being formed on a semiconductor substrate. In this semiconductor device, division regions are formed in part of the first semiconductor portion in such a manner as to cross the first semiconductor portion and partially enter the second semiconductor portion, so that the junction is divided into a plurality of parts by the division regions, to form a plurality of photodetector regions having the divided junction parts; and when a reverse bias voltage, which is equal to or less than a specific reverse bias voltage applied to the divided junction parts upon operation of the photodetector, is applied to the divided junction parts, depletion layers originated from two divided junction parts, disposed on both the sides of each of the division regions, of the plurality of divided junction parts extend, in the second semiconductor portion, under the division region to be brought into contact with each other.
With this configuration of the above semiconductor device, when a reverse bias voltage equal to or less than the specific value is applied to each of the divided p-n junction parts, the adjacent depletion layers via each division region extend under the division region to be brought into contact with each other, and accordingly, the division region between the photodetector regions becomes shallower than the prior art division region which includes the buried isolation region. As a result, the absorption wavelength of light incident on this division region is reduced, to enhance the light receiving sensitivity of the photodetector.
Further, when a reverse bias voltage equal to or less than the specific value is applied to each of the divided p-n junction parts, the adjacent depletion layers via each division region extend under the division region to be brought into contact with each other, and accordingly, it is possible to avoid the occurrence of such a potential barrier of the division region as to cause the roundabout of carriers generated under the depletion layer by light irradiation, and hence to avoid the degradation of the frequency characteristic due to the roundabout of the carriers.
According to a second aspect of the present invention, there is provided an optical pickup system including: a semiconductor light emitting device; a photodetector having a junction at which a first conductive first type semiconductor portion and a second conductive second type semiconductor portion are joined to each other, said photodetector being formed on a semiconductor substrate; and an optical system. In this photodetector, division regions are formed in part of said first semiconductor portion in such a manner as to cross said first semiconductor portion and partially enter said second semiconductor portion, so that said junction is divided into a plurality of parts by said division regions, to form a plurality of photodetector regions having said divided junction parts; and when a reverse bias voltage, which is equal to or less than a specific reverse bias voltage applied to said divided junction parts upon operation of said photodetector, is applied to said divided junction parts, depletion layers originated from two divided junction parts, disposed on both the sides of each of the division regions, of said plurality of divided junction parts extend, in said second semiconductor portion, under said division region to be brought into contact with each other.
With this configuration of the optical pickup system, since the semiconductor device is so configured as described above, it is possible to realize a good frequency characteristic and a good light receiving sensitivity even if an area including isolation regions for isolating a plurality of divided photodiode parts from each other is irradiated with light to obtain an RF signal requiring a high speed performance, or a very thin stripe pattern or a thick pattern of the photodiode, typically used for a so-called laser coupler, is irradiated with light.
According to the present invention, the semiconductor device can be fabricated by the steps, the number of which is substantially the same as that of the steps of the prior art fabrication method, and accordingly, the optical pickup system using the semiconductor device can be fabricated by the steps, the number of which is substantially the same as that of the steps of the prior art fabrication method.